Crustaceana, 12 (1-3), pp. 74-86, 1967
ON THE TAXONOMY OF THE GENUS
CORNIGERIUS (CLADOCERA, POLYPHEMIDAE)
PH. D. MORDUKHAI-BOLTOVSKOI
Institute of Inland Water Biology of the Academy of
S c i e n c e s of t h e U . S.S . R . , B o rok , distr. Nekouz, U.S.S.R.
N. Pengo described in 1879 a new genus and species of
polyphemid Cladocera from the Sea of Asov under the name of
Corniger maeoticus. Although the very appropriate generic
name Corniger — the genus is distinguished from the rest of
the family by horn-like diverging processes on the head —
has been used several times in the literature on Cladocera,
it cannot be maintained since it is preoccupied by Corniger
Agassiz (1831) for a genus of fish and by Corniger Bohm
(1879) for a genus of Pycnogonida. Much to my regret I have
to propose a new name to replace Corniger Pengo, 1879, and
I suggest Cornigerius nom. nov. I am much indebted to Dr.
J. H. Stock for this indication and for aid in the choice
of a new name.
Zernov (1901) confirmed the existence of the species
C. maeoticus, and described briefly two more species of the
genus, namely C. horribilis and C. bi-cornis. A new
polyphemid obviously close to C. maeoticus was described by
G. O. Sars (1902) from the Caspian Sea but referred by him
to the genus Evadne under the name E. hircus. Sars denied
the independence of the genus Corniger and named Pengo's
species Evadne cornigera.
Meissner (1908), emphasizing the close resemblance of
the two forms, reduced the Caspian E. hircus to the status
of a "local variety" of the species from the Sea of Asov
and named it E. maeotica (Pengo) var. hircus (G. O. Sars),
while
Corniger
horribilis
described
by
Zernov
was
considered by him not to be a distinct species but merely a
form of E. maeotica.
Later on Behning (1938) reestablished the genus
Corniger but in a different scope. As the most distinctive
character of this genus Behning considered the strongly
developed and diverging caudal claws.
In his opinion the processes on the head — "horns",
though present in the majority of the species of this
genus, may be absent, as in the new species described by
him as Corniger glabriceps. Behning intended to give a
detailed description of this genus and all its species in
his revision of the whole group of the Caspian polyphemids.
This revision was not published, but the conclusion may be
drawn from his papers on other topics (Behning 1938, 1941)
that he suggested five species, namely Corniger maeoticus
(with some forms), C. glabriceps, C. auritus (mentioned but
not described by him), C. lacustris and a fifth one
(unknown). C. lacustris was described by Spandl (1923,
1924) under the name Evadne lacustris.
In my review of the Caspian fauna (MordukhaiBoltovskoi, 1960) I included in the list of Caspian species
neither Corniger auritus (since it is a nomen nudum) nor C.
bicornis
(since
this
species,
very
unsatisfactorily
described in 1901 by Zernov, was not found thereafter
during sixty years). I also refrained from the restoration
of the genus Corniger and thus listed only Evadne maeoticus
(in the Pontoasov basin), E. m. hircus, and E. glabriceps
(in the Caspian Sea).
However, the more detailed study of the pontocaspian
polyphemids led me to the conclusion that the genus
Corniger is to be restored, but rather in the sense of
Pengo than that of Behning. A common character of all
species of this genus is in my view the presence of
appendages on the head ("horns"). Therefore Corniger
glabriceps should not be included in this genus (it belongs
probably to the genus Podonevadne). On the other hand
Corniger bicornis Zernov, found by me in the Caspian Sea,
is a good species of the present genus. In this paper the
diagnosis of the genus Cornigerius is given, as well as the
description of all four species belonging to it, one of
which is new. C. maeoticus and C. bicornis are redescribed
in more detail than by previous authors.
Cornigerius nom. nov. (pro Corniger Pengo, 1879, preocc.)
Type species: Corniger maeoticus Pengo, 1879.
Diagnosis. — Head on the frontal surface1) with two
appendages developed to various degrees, from a small
spine-like projection to long horn-like processes bent on
the end. Depression behind head also variable and sometimes
hardly noticeable. Shell more or less elongate, elliptical
or tapering to end but never quite round.
The caudal claws are always very well-developed, and
diverging. The armament of the exopodites fits generally
the setal formula 2.2.2.1, i.e., as in the majority of
Caspian podonids, and the structure of the legs closely
resembles that in Podonevadne camptonyx Sars. However, in
one species (C. bicornis) this formula is variable, the
lesser seta of the exopodite of the second and third pair
being underdeveloped, sometimes lacking.
At present four species are known; three of these are
endemic in the Pontocaspian basin.
1
) On the figures the front side is situated above, the
hinder part below, as usual in the representation of podonids
(= polyphemids without caudal appendage; see MordukhaiBoltovskoi, 1965), i.e., in a swimming position.
The distance from the lower edge of the head to the top
of the shell (which covers only the brood pouch and heart and
may be named the incubatory chamber), called the length by
many authors, is properly the height and does not coincide
with the morphological length, i.e., the distance from the
frontal edge of the head to the hind edge of the postabdomen
or caudal claws.
Key to the species of the genus Cornigerius
1.
Head with one strong process (the "trunk") directed forwards and ramifying in two smaller processes
(the "horns").......................2
—
Head with two processes either immediately on its
frontal surface or on the rounded prominence of its
frontal surface....................3
2.
Horns usually very large and curved. Caudal claws
very long, strongly diverging (usually more than at
right angle), smooth, often curved.................
.......C. maeoticus (Pengo)
— Horns very small, not curved. Caudal claws slightly
diverging, covered with tiny spinules................
....................C. lacustris (Spandl)
3.
The frontal processes are very small, like
spines or knobs, and set widely apart. Caudal claws
strongly diverging. Setal formula of exopodite always
2.2.2.1..........................C. arvidi n. sp.
—
The frontal processes may be developed in various
degrees, their bases being close together and situated
on the prominence of the head. Caudal claws comparatively slightly diverging. Setal formula of exopodite
variable..............С. bicornis (Zernov)
Cornigerius maeoticus (Pengo, 1879)
Corniger maeoticus Pengo, 1879; Zernov, 1901.
Evadne maeotica, Sars, 1902; Meissner, 1908; Behning, 1941;
Mordukhai-Boltovskoi, 1960.
Female. — The head bears on its frontal surface one
strong thick process — the "trunk", dividing into two
"horns". Caudal claws always very long, strongly diverging,
usually curved on the top. Shell more or less elongated,
elliptical or pointed on the top. Armament of exopodites
fits the formula 2.2.2.1. According to their structure and
armament the legs are similar to those of Podonevadne
camptonyx, differing in having somewhat greater armament of
the exopodite of the first pair (a bundle of hairs on the
hind edge), the endopodites of the first to third pairs and
the maxillar appendages (groups of setae and hairs on the
sides). Moreover, the smaller of the apical setae of the
first endopodal segment of the first leg and of the last
endopodal segment of second and third legs is longer and
does not differ markedly from the rest of the setae, and
the smallest seta on the exopodite of the third leg is also
longer than in P. camptonyx. The maxillar appendages of the
second and third legs bear two dentiform spines each, the
distal one being the shorter in the second and the longer
in the third leg, and a conical sensitive seta (sensilla)
on the lateral edge (figs. 9-11). The legs of the fourth
pair, like in other podonids, are small, their exopodite is
reduced to a minute nrominence bearing a seta.
One or two large latent eggs (winter-eggs covered
with a hard thick shell) develop in the brood pouch of
gamogenetic females; the ephippia are not produced as in
all polyphemids.
Male. — Differs from the female, as in all
polyphemids, in having two copulative appendages (penes),
hooks on the distal segment of the first leg, and a larger
head (due to the better developed eye). Moreover, the
processes on the head and caudal claws are noticeably
shorter than in the female.
С. maeoticus has a subspecies in the Caspian and
another in Pontoasov basins, both subspecies forming a
similar deviating form (aberratio).
Key to the subspecies and forms of Cornigerius maeoticus
( P e n g o ) , b a s e d on the females
1.
Horns of the head long, strongly diverging, curved at
the top, separated by a rounded interspace ...........2
—
Horns of the head short, nearly straight, separated
by an angular interspace..... C. maeoticus abb. horribilis
(Zernov)
2.
Shell
oviform,
rounded
on
top.
Pontoasov
basin.................. C. maeoticus maeoticus (Pengo)
—
Shell tapering towards the top, usually pointed.
Caspian Sea ...............C. maeoticus hircus (Sars)
Cornigerius maeoticus maeoticus (Pengo)
Corniger maeoticus Pengo, 1879; Behning, 1941.
Evadne cornigera Sars, 1902.
Evadne maeotica Meissner, 1908; Mordukhai-Boltovskoi, 1960.
Female. — Horns on the head usually strongly
chitinized and often coloured brown (as well as the top
part of the trunk), always pointed, curved at the ends and
strongly diverging, divided by an arcuate interspace. They
vary in form, being often curved and bearing on the end
several
minute
denticles
(figs.
1-4).
Occasionally
anomalies occur, e.g., underdevelopment of one horn. The
shell is evenly rounded, comparatively short, and as long
as wide at the base.
Adult parthenogenetic females measure between 0.55
and 1.1 mm; being usually 0.7 to 8 mm.
The parthenogenetic female has been described in
detail by Pengo (1879), though he figured it in an
unnatural position.
Gamogenetic females, i.e. those bearing the one or
two latent eggs, do not differ from the parthenogenetic
females in their appearance. They occur rather rarely and
as single specimens.
Male. — The males differ from the females, in
addition to the above mentioned characters, in the shape of
the shell, tapering towards, though rounded at the top.
Males are somewhat smaller than females, measuring 0.6 to
0.7 mm. They were found first in August 1963 in the Sea of
Asov.
Distribution. — Sea of Asov, mainly in slightly
brackish (oligo- and meiomeso-haline) waters (Gulf of
Taganrog and northeastern part of the open sea); Dnje-proBug liman and limans in the Danube delta. It occurs also in
quite fresh waters and in the Danube River; during the last
years it appeared in the Kachovskoie reservoir of the
Dnjepr and near Zimlianskoje on the Don (MordukhaiBoltovskoi, 1964).
Cornigerius maeoticus hircus (Sars)
Evadne hircus Sars, 1902, 1927.
Corniger maeoticus hircus Behning, 1941.
Evadne maeotica hircus Mordukhai-Boltovskoi,
1960.
Female. — Shell more or less triangular, tapering to
the end and usually pointed, 1.5 to 2 times longer than
broad at the base. Pointed top of the shell often slightly
bent and directed somewhat forward. In the structure of the
head appendages and the legs it does not differ noticeably
from C. m. maeoticus (figs. 5-11).
The adult parthenogenetic females measure between 0.7
and 1.3 mm, i.e. somewhat more than the above-mentioned
subspecies, but this is on account of the more elongated
shell only.
Gamogenetic females occur rarely. I found them in the
autumn only, my material consists of single specimens with
one or two winter eggs.
Male. — The male differs from the female as in the
typical subspecies. The shell is pointed as in the female
but its top is always directed upwards (MordukhaiBoltovskoi, in press). They are rare, and until now have
been found only in the central Caspian Sea: one specimen in
May 1963 and many in only one of the samples taken in
September 1963.
Distribution. — Caspian Sea. Throughout the whole
sea, including the freshened northern part.
Cornigerius maeoticus aberratio horribilis (Zernov)
Corniger horribilis Zernov, 1901.
Evadne maeotica Meissner, 1908.
Corniger maeoticus horribilis Behning, 1941.
Female. — The female differs from the typical form of
both subspecies in the armament of the head only. The horns
are markedly shorter than in the typical form, they are
separated from each other by an angle, which is often less
than 90 degrees. The horns are straight or nearly straight
and have sometimes blunt tips and a rugged or wrinkled
surface (figs. 12-17).
This form was undoubtedly found first by Pengo, who
mentioned the existence, besides the typical С maeoticus,
of specimens with "short, straight and as if twisted
horns". Zernov (1901) found them anew and described them as
a distinct new species under the name Corniger horribilis.
However, Meissner (1908) denied the independence of this
species. Indeed, specimens occur in the Sea of Azov with
horns to a certain extent transitional to the horns of
typical form. Otherwise, no marked difference exists
between the two forms either in the shape of the shell or
in other characters including the dimensions (0.6-0.7 mm).
This form occurs in the Sea of Asov as well as in the
Caspian Sea and may be ranked not more than a variety or,
at best, an aberration.
Cornigerius lacustris (Spandl)
Evadne lacustris Spandl, 1923, 1924; Mordukhai-Boltovskoi, 1960.
Corniger lacuslris Behning, 1941.
Female. — The long conical process of the head, the
"trunk", bears two short horns, which are rather markedly
set off from it. Shell tapering to the top and usually
pointed, more rarely rounded.
Caudal claws strong and long, but slightly curved and
diverging, covered in distal part with numerous minute
spinules (figs. 18-20, from Spandl). Females measure 0.600.75 mm; the male is still unknown.
Distribution. — This species was found in June 1914
by Spandl in a mountain freshwater lake, Lake Goldjik, in
Turkey.
Remarks. — Judging from the description of Spandl,
which is somewhat insufficient although based on numerous
specimens,
C.
lacustris
differs
distinctly
from
C.
maeoticus. However, the author suggested the possibility
that they may represent extremes of a single variable
species.
Unfortunately С. lacustris has not been studied
thoroughly. So far its single known locality is lake
Goldjik in the basin of the Euphrates. This is also the
single known occurrence outside the Ponto-Aral-Caspian
basin of a polyphemid, which undoubtedly belongs to the
Caspian complex.
Cornigerius arvidi n. sp.
Holotype. A parthenogenetic female in the collection of the
Zoological Institute of the Academy of Sciences of the USSR, No.
46734. Locality: Central Caspian Sea between Makhach Kala and
Sagindik, depth 46 m, 28 August 1943.
Paratypes. Several parthenogenetic females from the same sample
and from other samples, taken in the Central Caspian in 1958,
deposited in the same collection as paratypes, No. 46735.
Description of female. — Head large and widened at
both sides. Lateral areas of the head prominent, resembling
angular knobs, usually bearing small spiniform processes,
sometimes unequal in size. Though the spiniform processes
are often lacking, the angular knobs are always present and
readily visible when the animals are viewed from the back
(figs. 22-25).
At its base the shell is swollen, it strongly tapers
toward the top, and is nearly always pointed; in females
with developed embryos it becomes more bulging though it
preserves essentially the same shape.
Caudal
claws
very
long,
curved
and
strongly
diverging, mostly to opposite sides (figs. 21, 22).
Legs built very similar to those of C. maeoticus.
Armament of exopodites showing setal formula 2.2.2.1.
Maxillar appendages of second and third legs less hairy,
but dentiform spines on their tops larger than in С.
maeoticus (figs. 26-28).
Parthenogenetic females measure 0.65 to 0.70 mm;
gamogenetic females and male are still unknown.
Distribution. — Central and southern Caspian Sea.
Discovered first in samples taken in 1913, collected again
between 1940 and 1950.
Remarks. — This species is named in honour of Prof.
Arvid Behning, outstanding carcinologist, who greatly
advanced our knowledge of the fauna of the Caspian basin.
While giving a brief characterization of the genus
Corniger, Behning (1938) wrote: "The majority of species
[of this genus] bear processes on the upper surface of the
head, from little spinules (C. auritus) to branched horns
(C. hitcus)". Perhaps Behning implied under the name C.
auritus the above species. However, the lack of figures and
of any satisfactory description in Behning's paper prevents
the identification with these names 2).
Cornigerius bicornis (Zernov)
Corniger bicornis Zernov, 1901.
Female. — The appendages on the head are represented
by two horns, issuing separately from the frontal surface
or, more often, from a rounded bulge or low knob. A
distinct "trunk" never exists. The shape and size of these
horns, as shown in figs. 29-33, are extremely variable,
ranging from slightly obtuse knobs to rather long pointed
processes sometimes unequal in length.
The shell is always more or less elongated, tapering
to the top, but also very variable in shape. The external
appearance of the animal therefore is very changeable,
varying from comparatively large specimens with a nearly
oviform body and a very low head to considerably smaller
ones with a shortened nearly triangular shell and an
elevated head.
The caudal claws are strongly developed but as a rule
slightly curved and not much diverging (at an angle of less
than 90 degrees), by which the species under description
differs from the rest of the Cornigerius species. Another
important difference of С. bicornis consists in the partial
reduction of the exopodite setae of the second and third
legs. A part of my individuals has the usual expodite
armament of 2.2.2.1, but in many others a seta, namely the
lesser one, is undeveloped and very thin or lacking on the
third and sometimes also on the second leg, the exopodite
armament then fitting the formulae 2.2.1.1 or even 2.1.1.1
(figs. 34-36), i.e., like Podonevadne trigona. Sometimes
specimens occur in which there is a different setal formula
on the right and left legs. Thus the exopodite armament in
this species is unstable, С. bicornis occupying in this
respect a particular position in the family Polyphemidae,
where the setal formula is usually considered a good
distinctive specific character. Parthenogenetic females
measure 0.5 to 0.7 mm; gamogenetic females and males are
unknown.
Distribution. — Caspian Sea and Sea of Asov.
Remarks. — C. bicornis was found first by Zernov
(1901) in the Sea of Asov in May 1900. In his description
Zernov limited himself to the indication of two horns,
issuing directly from the head (not from a common trunk as
in the rest of Cornigerius species). Zernov added that he
saw this species twice only and could not study its
structure in detail.
So far no indications of this species have been
published after the appearance of Zernov's paper.
In looking through the plankton samples taken in the
Caspian Sea in May 1913, I unexpectedly discovered in
rather great numbers a polyphemid corresponding to the
above given description. The resemblance in the situation
of head processes and in the body shape with Zernov's
description, in spite of the incompleteness of the latter,
puts beyond doubt the identity of this form with
Cornigerius bicornis (Zernov). Later on the species was
found (in two specimens only)
by L. Butorina in samples
taken in the Central Caspian in May 1963.
2
) In my preliminary review (Mordukhai-Boltovskoi, 1964) I
listed С. auritus Behning with the notation that it was not
described. Later on it turned out that in one more species of
Corniger, namely С. bicornis (described below) the processes on
the head are also often reduced to little knobs. So it is
difficult to determine which species was named by Behning C.
auritus and it would be best to ignore this nomen dubium.
Judging by the fact that in summer and autumn samples
from the Caspian and Asov Seas C. bicornis was never found,
the assumption arises that this species is one of the very
few polyphemids connected with the spring season.
CONCLUSIONS
All the species and subspecies of the genus
Cornigerius have a common character, viz., appendages on
the head, distinguishing them from the rest of polyphemids
(excepting Caspievadne, which has in addition, however, two
large processes on the shell). However, with respect to the
structure of the legs and the caudal claws the species of
Cornigerius are not quite uniform and may be divided into
two groups: (1) C. maeoticus (with all its forms) and C.
arvidi, in which the exopodite armament formula is 2.2.2.1
and which have strongly diverging caudal claws, and (2) C.
bicornis and C. lacustris, in which the caudal claws do not
strongly diverge; in the first species of the second group
the exopodite armament is unstable, in the second the setal
formula is 2.2.2.1. A more detailed study of the latter is
very desirable and would help to clear up the relationship
of Cornigerius with other polyphemids.
ZUSAMMENFASSUNG
REFERENCES
BEHNING A.L., 1938. О zimnem zooplanktone Kaspiiskogo moria.
Trudy po kompleksn. Izucheniu Kasp. moria, 5: 7-97.
[in Russian].
_______, 1941. Cladocera Kawkaza. Vysokogorn. biolog.
stancia Gruzii: i-vii, 1-384. [in Russian].
MEISSNER V., 1908. Mikroskopische Wassertiere des Aralsees und
der einmiindenden Fliisse. Wissen. Ergebn. AralseeExped., 8: 1-102, pls. 1-4. [in Russian].
MORDUKHAI-BOLTOVSKOI PH.D., I960. Kaspiiskaia fauna w AzovskoChernomorskom basseine: 1-288. (Moskwa, Akad. Nauk).
[in Russian].
_______, 1965. Polyphemidae of the Pontocaspian Basin.
Hydrobiologia, 25 (1-2): 212-220.
_______, in press. On the males and gamogenetic females of
Caspian Polyphemids. Crustaceana.
_______,
in
press.
О
Kaspiiskih
Polyphemidae
w
wodohranilishchah Dona i Dniepra. [in Russian].
PENGO N., 1879. О nowom rakoobraznom iz sem. Polyphemidae.
Trudy Obshch. Ispytat. Prirody Harkow. Univ., 13: 920, pi. 1. [in Russian].
SARS G.O., 1902. On the Polyphemidae of the Caspian Sea.
Ann. Mus. zool. Ac. Sc. St.-Petersb., 7: 31-54.
_______, 1927. Notes on the Crustacean fauna of the Caspian
Sea. Festschrift fur Prof. N. Knipowitsch: 315-329.
SPANDL H., 1923. Zur Kenntnis der Siisswassermikrofauna
Vorderasiens. Ann. nat. Mus. Wien, 36: 124-149.
_______, 1924. Hydrobiologisches aus Armenien. Verh.
intern. Verein. LimnoL, 1: 249-251.
ZERNOV S.A., 1901. Resultaty zoologich excursii po Azovskomu
morju 10-20. V. 1900. Ann. Mus. zool. Acad. Sci. St.Petersb., 6 (4): 559-584. [in Russian].
Received for publication 8 September 1964.